This note examines the impact of district magnitude on political party fragmentation in subnational Indonesia. The analysis employs regression discontinuity methods to accommodate the endogeneity of legislature size and identify its causal effect on the number of political parties represented in the local council. While the potential endogeneity of district magnitude has long been recognized in the literature, this appears to be the first study in which it has been empirically modelled as endogenous. The examination finds that rising endogenous district magnitude leads to an increase in the number of political parties in the legislature, as standard theory predicts. More significantly, the investigation determines that the positive impact of legislature size on party fragmentation is most pronounced for those local administrative units that have been recently established and whose governance environments are particularly fragile, suggesting that the broader institutional context is especially important in conditioning district magnitude effects.